The present invention relates to two-stage radial compressors and to turbochargers that include such compressors.
Radial compressors are used in various types of turbomachinery, including turbochargers for internal combustion engine systems. A radial compressor generally includes at least one compressor stage formed by a rotating impeller mounted on a shaft within a compressor housing. The housing defines an inlet flow path that typically leads into the impeller in a generally axial direction. The impeller includes a hub and a plurality of blades spaced about its circumference and extending out from the hub. The impeller is configured to receive fluid in the axial direction and to compress the fluid and discharge the fluid in a generally radially outward direction into a volute defined by the compressor housing. The housing includes a wall or shroud that extends proximate the tips of the impeller blades and, together with the hub of the impeller, defines the main flow path through the impeller.
In some applications requiring pressure ratios above that achievable by a single-stage radial compressor, two-stage radial compressors are employed. A second stage is formed by a second-stage impeller, which receives the fluid from the first-stage impeller and further compresses it to a higher pressure. Examples of two-stage radial compressors are described in U.S. Pat. Nos. 6,062,028 and 6,834,501, the disclosures of which are incorporated herein by reference.
In any compressor, there is a limit to the pressure ratio that can be attained at a given flow rate before surge occurs; the locus of points at which surge occurs, as a function of flow, is referred to as the surge line on the compressor map. There is also a limit to how much flow can be passed through the compressor before choking occurs somewhere in the compressor. The useful operation range of the compressor is defined between the surge line and the flow rate at choke. It is desirable to have a wide range of operation. In particular, on a compressor map of pressure ratio versus flow rate, it is desirable to push the surge line as far toward the upper left-hand corner of the map as possible. Many different approaches for controlling surge in compressors have been proposed over the years. Some approaches involve relatively complicated active control systems using feedback control techniques and/or variable-geometry mechanisms in the compressor. For many applications, such as turbochargers, such complex approaches are not practical.